A digital compound image includes mixed raster content (MRC) such as some combination of text and picture image(s) and/or graphic(s). Exemplary compound images include, for example, screen captures, electronic newspapers and magazines, web pages, etc. With the widespread use of digital devices such as personal computers, digital cameras, imaging-enabled cell phones, etc., availability and sharing of compound images is becoming more common. To store and communicate digital compound images, a compound image is typically compressed (encoded) to reduce size. The quality requirement of compound image encoding is generally different from the quality requirement for encoding images that do not contain text (a general/non-compound image). This is because sensitivity of the human eyes for natural images (e.g., captured images, etc.), texture, text and other sharp edges, is often different. While there may be several acceptable levels of pure image and texture quality, a user will typically not accept text quality that is not clear enough to read. This is because text typically contains high-level semantic information.
The Lempel-Ziv encoding algorithm is designed to compress pure text images (e.g., images with only text on the pure color background). JPEG image encoding is suitable for images that include only pictures and no text. One reason for this is because JPEG encoding algorithms typically do not perform very well when encoding text. Additionally, existing compound image encoding techniques such as layered coding techniques do not typically perform well when encoding pure text images. Layered encoding techniques are also very processing intensive, making them generally unsuitable for real-time applications that demand constant bit-rate encoding and rate allocation (e.g., for streaming compound image content). Moreover, conventional block-based compound image encoding typically compress text blocks using JPEG-LS and picture blocks using JPEG. Such block-based encoding techniques fail to perform well when encoding compound images that include some combination of text and picture image(s).